In conventional projection televisions, a projection tube is mounted on a frame having a window that opens to the front of the fluorescent screen of the tube. A packing is inserted between the peripheral edge of the window and the front face of the tube. A first lens is mounted in the window in the mounting frame with a packing inserted therebetween. A liquid coolant is contained between the first lens and the fluorescent screen of the projection tube. The mounting frame is also fitted with a lens frame in which is typically mounted at least one lens for projecting an image on the fluorescent screen onto a large-size screen.
When the first lens is mounted on the mounting frame, installation errors can cause misalignment between the centers of the window in the frame and the center of the first lens. Misalignment can also occur between the centers of the lens frame and the window when the manufacturer installs the lens frame on the mounting frame. The second alignment error compounds the first alignment error with the result that the center of the first lens becomes offset from that of the lens frame. This will cause the picture on the projection screen to lose sharpness.
To avoid this problem, it is necessary to attain alignment between the centers of the lens frame and the first lens. This alignment step must be done by the operator while he is looking at the picture on the screen to make sure that it is in focus. This not only requires great skill but also involves considerable difficulty.